[Bugfix][Parser] Coerce nested strings in decoded XML arguments

[Dreamer-HIT]

Purpose

The XML tool parser decodes an object or array parameter from a string, but
ParserEngine._coerce_value immediately returns that container without applying
the existing properties/items recursion. For an inline schema declaring an
array of integers, <parameter=limits>["10", "20", "30"]</parameter> therefore
produces string elements. Nested numeric, boolean and nullable fields have the
same problem, including in streaming responses. Clients expecting the declared
argument types can reject these tool calls.

After a string successfully converts to a dict or list, recurse through its
properties/items and coerce nested strings. An internal
coerce_native_scalars flag keeps the decoded container's existing JSON
numbers, booleans and nulls unchanged. Without that distinction, the existing
fallback to string for untyped schemas would turn {"n": 1} into
{"n": "1"} under properties: {n: {}}, introducing a regression.

Existing native scalar coercion remains enabled by default for native JSON
input. Stringified container children in such input also gain the new
recursion. String conversion, union precedence and best-effort failure behavior
are unchanged.
This is a nested-string correction, not full schema normalization: a decoded
native 1 still remains 1 even if its schema requests string, as it did
before. No JSON Schema keyword support is added.

Extend the existing Qwen3 parser tests to cover both native JSON values and
string values requiring nested conversion. The existing string-array guard
continues to ensure that values such as "42" stay strings when requested.
Add native scalar preservation cases for objects and arrays with empty,
description-only, const, and explicit string schemas.

Duplicate-work check

This is the independent recursion bug explicitly identified and deferred by
the author of #50933 in this comment,
while investigating #46924. It reproduces with inline schemas and does not
implement the $ref/$defs resolution in #50933. #54964 fixes a separate
Hermes/Granite outer-arguments double-encoding problem. This PR does not claim
to close #46924.

Read #46924 and its comments and searched open PRs for 46924 in:body,
nested coercion, XML recursive, and _coerce_value before preparing this
change.
Checked the actual patches of #53729, #51577 and #49318 as well. #53729 changes
adjacent coercion statements for schemas without concrete types but retains
the string-to-container early return. This change avoids depending on its
type-inference changes by preserving decoded native scalars. #51577
migrates Llama JSON parsing and still delegates string coercion to this method.
#49318 adds Kimi argument conversion and tests native nested objects, without
changing this string-to-container path.

Test Plan and Results

Environment: WSL Ubuntu 24.04, Python 3.12.14, PyTorch 2.13.0+cu132. Python source
base: 569adb5a9780f9c02d22a6b29826acf711512356. Parser tests use CPU and the
existing mock-tokenizer fixtures; they exercise the public streaming and
non-streaming parser entry points.

.venv/bin/python -m pytest tests/parser/engine/test_qwen3.py \
  -k TestNestedSchemaCoercion -q --tb=short
.venv/bin/python -m pytest tests/parser/engine -q --tb=short

• Original five nested-schema tests: 5 passed. Their container contents
  already had the expected JSON types, so they did not catch the missing step.
• Same expanded nested-schema cases, before the fix: 4 failed, 5 passed.
  Failures cover numeric object fields, numeric array items, and boolean/null
  fields in an array of objects in non-streaming and streaming responses.
• With the final fix, the entire parser engine suite: 3855 passed,
  including 24 native-scalar preservation cases.

Model smoke check: Qwen2.5-0.5B-Instruct at revision
7ae557604adf67be50417f59c2c2f167def9a775, FP16 on an RTX 4060 Laptop 8 GB,
eager TRITON_ATTN, max model length/batched tokens 1024, max sequences 2,
memory utilization 0.55, prefix caching off, seed 0, temperature 0,
256 output-token limit. V1 model runner and PyTorch sampling were selected
for WSL compatibility.

Generated seven XML calls and replayed each exact text and generated token-ID
sequence through the public ParserEngine.parse before and after the change.
The baseline restores both _coerce_value and _coerce_dict from the source
base above. No generated output was edited or replaced.

• Tool calls matching the expected values and schema: 2/7 before, 5/7 after.
• Three generated outputs changed from failure to success: two integer arrays
  containing quoted numbers, and an object with min_stars: "125".
• No previously successful call regressed. The string-array control and the
  native-scalar object with empty/const schemas stayed correct.
• Two generations remained unsuccessful in both versions: malformed XML for
  the boolean/null array, and an untyped array where the model emitted "1"
  instead of the requested native 1. One intended native-integer control
  also emitted quoted numbers and therefore became a fix witness instead.

This is a targeted shared-parser smoke check using prompted Qwen2.5 outputs,
not a Qwen3-Coder evaluation or a general accuracy/throughput benchmark.
Boolean/null coercion and native-array preservation are covered by the unit
tests above.

Runtime provenance: imported this checkout via explicit PYTHONPATH, with
official cu130 native artifacts for the exact source base above. An editable
packaging pass was stopped after artifact extraction because WSL file scans
were slow; existing package registration was retained. New and old dependency
requirements matched, and the actual source/extension import paths and CUDA
availability were verified before the smoke run.

Applicable pre-commit hooks and the additional Python 3.12 mypy hook passed
on both changed files:

SKIP=actionlint pre-commit run --files \
  vllm/parser/engine/parser_engine.py tests/parser/engine/test_qwen3.py
SKIP=actionlint pre-commit run mypy-3.12 --hook-stage manual --files \
  vllm/parser/engine/parser_engine.py tests/parser/engine/test_qwen3.py

The unused actionlint hook was excluded because no workflow YAML changed
and its Go environment bootstrap previously timed out on this host.

All applicable commit hooks and the DCO sign-off hook passed during creation
of the final commit. The separate final Python 3.12 mypy run also exited 0.

AI assistance: Codex assisted with issue triage, diagnosis, implementation,
regression tests and local validation. Commits include AI attribution and DCO
sign-off.

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Reviewing files that changed from the base of the PR and between 808f8cd and f3d11ec.

📒 Files selected for processing (2)

• tests/parser/engine/test_qwen3.py
• vllm/parser/engine/parser_engine.py

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved schema coercion for nested objects and arrays containing string-encoded numbers, booleans, and null values.
  • Preserved native scalar types when processing decoded JSON containers.
  • Applied consistent behavior across streaming and non-streaming responses.

Walkthrough

The parser now preserves native scalar types while coercing nested string values inside decoded containers. Qwen3 tests cover object, array, streaming, boolean, null, numeric, and schema-kind combinations.

Changes

Scalar Coercion

Layer / File(s)	Summary
Recursive coercion control vllm/parser/engine/parser_engine.py	_coerce_value and _coerce_dict propagate native-scalar handling through nested objects and arrays.
Native and encoded scalar coverage tests/parser/engine/test_qwen3.py	Tests cover native and string-encoded numbers, booleans, and null values in nested and streaming payloads. Additional cases verify that decoded containers preserve native scalar types.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to f3d11

Decoded tool-call containers now coerce nested string values according to inline schemas while retaining native JSON scalar types. Nested, streaming, and scalar-preservation coverage is present, with no identified merge-blocking risk.

🚥 Pre-merge checks | ✅ 2 | ❌ 3

❌ Failed checks (3 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The pull request fixes inline decoded-container recursion, but linked issue #46924 requires $defs/$ref resolution or normalization, including regression coverage for referenced schemas. The descriptio…	Implement root-schema $ref/$defs resolution or consistent dereferencing, preserve structured nested output for referenced schemas, and add regression tests. Alternatively, link this pull request to an issue that covers the inline-schema rec…
Out of Scope Changes check	⚠️ Warning	The parser changes and Qwen3 tests target a separate inline-schema recursion issue. They do not address the linked issue's $defs/$ref requirements, so the main changes are outside the linked scope.	Either limit the pull request to changes required by #46924, including referenced-schema handling, or remove #46924 as the linked issue and associate the pull request with the correct inline-coercion issue.
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the parser bugfix and the nested-string coercion change for decoded XML arguments.
Description check	✅ Passed	The description directly explains the decoded-container recursion bug, the implementation, test coverage, and validation results.

Full details: Linked Issues check

Explanation

The pull request fixes inline decoded-container recursion, but linked issue #46924 requires $defs/$ref resolution or normalization, including regression coverage for referenced schemas. The description explicitly states that this requirement is not implemented.

Resolution

Implement root-schema $ref/$defs resolution or consistent dereferencing, preserve structured nested output for referenced schemas, and add regression tests. Alternatively, link this pull request to an issue that covers the inline-schema recursion fix.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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